(1) Field of the Invention
The present invention related to a display panel manufacturing method and an exposure system employed in the method. More particularly, the present invention is concerned with a technology that will prove effective when adapted to a method of manufacturing a thin-film transistor (TFT substrate to be included in a liquid crystal display panel.
(2) Description of the Related Art
In the past, liquid crystal display devices have been widely used as displays for televisions or personal computers or displays for portable cellular phones or handheld terminals (personal digital assistants).
The liquid crystal display device is a display device including a liquid crystal display panel that has a liquid crystal material sandwiched between a pair of substrates. Formed on one of the pair of substrates (hereinafter, referred to as a TFT substrate) are, for example, multiple scanning signal lines, multiple video signal lines that three-dimensionally intersect the multiple scanning signal lines with an insulating layer between them, and TFT elements and pixel electrodes each of which is disposed in a pixel area enclosed with two adjacent scanning signal lines and two adjacent video signal lines.
Assuming that the liquid crystal display panel adopts, for example, a vertical electric field driving method such as a twisted nematic (TN) method or a vertical alignment (VA) method, opposite electrodes (which may be referred to as common electrodes) opposed to pixel electrodes are formed on one substrate opposed to a TFT substrate. Moreover, when the liquid crystal display panel adopts, for example, a horizontal electric field driving method such as an in-plane switching (IPS) method, the opposite electrodes opposed to the pixel electrodes are formed on the TFT substrate.
For manufacture of the TFT substrate of the liquid crystal display panel, for example, a step of forming a thin film on a glass substrate and a step of etching the thin film are repeated multiple times in order to sequentially form patterns of scanning signal lines and others.
At the step of etching the thin film, first, a photosensitive resist film is formed on the thin film, and exposed to light according to predetermined design patterns. Thereafter, the exposed resist film is developed in order to produce etching masks. The etching masks are used to etch the thin film in order to produce thin-film patterns on which the design patterns are reflected.
In a conventional TFT substrate manufacturing process, when the resist film is exposed to light, a photo mask is generally employed. The photo mask is a mask having exposure patterns, which reflect the design patterns, formed on a glass substrate using a metal film made of, for example, chromium (Cr).
One of important points in manufacturing the liquid crystal display device is to homogenize image quality in a display field on a liquid crystal display panel. Namely, when the image qualities in areas within a display field on a liquid crystal display panel, for example, the image qualities in the center part of the display field and a corner thereof, the image qualities at the left end of the display field and the right end thereof, or the image qualities on the upper end of the display field and the lower end thereof are compared with each other, they have to be visually homogeneous.
However, among the conventional liquid crystal display devices, in large-size liquid crystal display devices, for example, liquid crystal televisions and liquid crystal displays for personal computers, it is hard to homogenize the image quality in the display field of a liquid crystal display panel. This poses a problem in that inhomogeneity in image quality occurs.
One of causes of the inhomogeneity in image quality in the display field of a liquid crystal display panel is a phenomenon referred to as source-drain (SD) thinning. The SD thinning is the phenomenon that the planar width of a source electrode of a TFT element formed by actually etching a conductive film or a drain electrode thereof gets smaller than the one designated in an associated design pattern.
When the width of an actually formed source electrode or drain electrode gets smaller than the one designated in an associated design pattern, for example, the channel width of a TFT element diminishes. A current value to be written in the TFT element gets smaller. This brings about insufficient writing of gray-level data. Consequently, for example, when a pixel at which the width of an actually formed source electrode or drain electrode is nearly identical to the one designated in an associated design pattern adjoins a pixel at which the SD thinning has occurred, inhomogeneity in luminance occurs near the border between the pixels.
Assuming that a source electrode of a TFT element or a drain electrode (video signal line) thereof is formed using an etching resist produced by performing exposure according to a conventional exposure method that employs a photo mask, when the phenomenon referred to the SD thinning occurs, an exposure pattern for a portion that has undergone the SD thinning has to be corrected in order to produce a new photo mask.
However, since the photo mask has to have exposure patterns thereof formed highly precisely, production of the photo mask requires much time and costs a lot. Moreover, the SD thinning does not always occur at the same location, but the location of the SD thinning varies depending on a condition for formation of a source electrode or drain electrode (video signal line). Consequently, the method of correcting any of the exposure patterns of the photo mask for the purpose of preventing the SD thinning is unfeasible.
Moreover, inhomogeneity in image quality occurring in a display field of a liquid crystal display panel is known to relate to a variance of the width of each scanning signal line formed on a TFT substrate or a variance of the width in one area of a scanning signal line, that is, variances of the dimensions of a pattern actually formed in each pixel area on a TFT substrate. Moreover, the variances of the dimensions of a pattern actually formed in each pixel area are known to relate to, for example, a variance in a film thickness in each area on the glass substrate occurring at the time of forming a thin film such as a conductive film or an insulating film on a glass substrate, or a variance in a magnitude of etching in each area on the glass substrate occurring at the time of etching the thin film. The variance in the thickness of the thin film or the variance in the magnitude of etching gets more obvious along with an increase in the size of the glass substrate, an increase in an area over which a film is formed at a time, or an increase in an area to be etched.
When it comes to the conventional exposure method using a photo mask, exposure patterns of multiple scanning signal lines formed in the photo mask reflect respective design patterns, and are produced so that the width of each scanning signal line and the space between two adjoining scanning signal lines will remain constant. Therefore, etching resists are formed so that, for example, even when a variance in the thickness of a thin film has occurred, all the scanning signal lines will have the same width. Consequently, a variance in a magnitude of etching occurs, for example, between a position at which the thickness of the thin film is small and a position at which the thickness thereof is large. This brings about a variance in the width of each scanning signal line. Namely, at the position at which the thickness of the thin film is small, the width of the scanning signal line gets smaller than that at the position at which the thickness of the thin film is large.
However, as mentioned above, formation of a photo mask requires much time and costs a lot. Moreover, a variance in the thickness of a thin film has the tendency that the thin film thickness gets larger near the center of a glass substrate but gets smaller toward the end thereof. The distribution of thicknesses has a variance. Moreover, for manufacture of a TFT substrate, for example, a method that uses a substrate having a large area and being referred to as a mother glass and is called a multi-plane production method is adopted. Therefore, even a TFT substrate has thin-film patterns formed using the same photo mask, the distribution of thicknesses of a thin film varies depending on to which of fields on the mother glass a position on the thin film belongs. Consequently, for example, even the method of correcting an exposure pattern on the photo mask for the purpose of coping with a variance in the thickness of the thin film or a variance in a magnitude of etching is unfeasible.
As mentioned above, as far as the conventional liquid crystal display panel is concerned, a resist film is exposed to light according to a conventional exposure method that uses a photo mask. Therefore, it is hard to control a variance of the width of each scanning signal line or each video signal line or a variance lf a current to be written in a TFT element which is derived from a variance in the thickness of a thin film or a variance in a magnitude of etching. This poses a problem in that it is hard to minimize inhomogeneity in image quality.